Sex Linkage & the Chromosome Theory - 05

what are the parallels between genes & chromosomes?

• both come in pairs

• segregate in meiosis & mitosis

• show independent assortment in the gametes

where did proof for the chromosome theory come from?

• Thomas Hunt Morgan - reciprocal crosses in Drosophila melanogaster

• recessive mutant turned up in one stock, changed eye colour from red to white

• reciprocal crosses gave a different result, depending on whether the male or female parent carried the mutant allele.

  • mated F1 flies together to get F2

which sex is homogametic/heterogametic?

• homogametic - females, two large X chromosomes

• heterogametic - males, a large X & a smaller Y

  • males are hemizygous

is the hypothesis that eye colour is carried on the X chromosome correct for white & red eye colour of Drosophila?

• pattern of inheritance of the white alleles follows exactly that of the X chromosomes

• strong evidence that genes are borne on chromosomes

• in some crosses, this pattern did not apply and instead had opposite results

• this is because females are aneuploids - an error during cell division, they produce eggs which are XX or have no sex chromosomes at all, causing a genetic abnormality in the stock.

what is sex-linked inheritance?

• Drosophila white eye colour - first example of this

• X-linked recessive alleles are carried through females, but only express themselves mainly in males

  • e.g. colour blindness!

• Haemophilia - factor VIII deficiency - blood can’t clot properly.

what is another example of an X-linked disease in humans?

Lesch-Nyhan syndrome

• fatal error in purine metabolism to self-mutilation and death

Colour blindness

• due to a fault in one of the opsin proteins, which are expressed in the retina

what is X-chromosome inactivation?

• if females have 2 X chromosomes, and males only have 1, it means females would be producing twice the amount of protein in males

• XCI equalises the dose of X-linked alleles in males & females, the Barr body (inactivated X chromosome) can be seen under a microscope

do females get colour blindness?

• if heterozygous for red-green colour blindness, they have a mosaic-like vision where they still have colour vision, it is just slightly impaired in the retina.

what is Duchenne muscular dystrophy?

caused by a mutation in the dystrophin coding gene which causes muscles to break down

• example of pleiotropy